The present invention relates to a method and an apparatus for polishing chamfers of a wafer.
1. Prior Art
A semiconductor wafer, which is used to make substrates for semiconductor devices, is obtained from a single crystal ingot, such as one grown of molten polycrystal silicon, by slicing the ingot into thin disks orthogonally to the axis of the ingot, which disks are then chambered, lapped, etched, annealed, polished, and given other finishing treatments.
Each semiconductor wafer, as produced in the manner generally described above, is chamfered along the peripheral edge thereof so as to remove any sharpness from the edge and thereby fashion the wafer edge difficult to chip. However, due to the increased demand for stricter prevention of dust creation called for by highly developed super LSI (large scale integration) technology based on high precision machining, the current tendency is to polish the chamfers of wafer periphery until they are glossy (specular finish polishing) so as to nullify the possibility of creation of particles due to microscopic chipping.
Incidentally, a wafer is usually formed with a cut-away portion where the wafer edge line is straight and this edge portion is conventionally called an orientation flat (OF for short), and when the wafer is chamfered and polished, the entire periphery including the OF is made glossy. Conventionally, the circular circumference portion as well as the straight-line OF portion of the wafer edge is polished by means of an external surface-contact type cylindrical polisher. FIG. 7 shows a manner of employing one of such external surface-contact type cylindrical polisher, which consists of a buff 140 made of a resilient body and formed with a groove 140a, which extends around the side of the cylinder describing a circle normal to the axis of the cylinder. The profile of the groove 140a is about complementary to the profile of the chamfered wafer edge so they fit each other when they are in contact.
The buff 140 is turned about the axis of rotation and the wafer being turned in the same angular direction as the buff 140 is brought so that the wafer edge enters the groove 140a. Slurry (polishing liquid) is applied to the running groove 140a, and the running edge of the wafer W is pressed in the groove 140a of the cylindrical buff 140, whereby the chamfers W1 of the wafer W are polished.
2. Problems the Invention seeks to solve
However, in such polishing method wherein an external surface-contact type cylindrical polisher (buff 140) is employed, the contact between the wafer edge and the side of the cylindrical buff is of convex-to-convex type so that the area of the wafer chamfers W1 contacted at any moment by the buff 140 is meager. Thus, the time efficiency of polishing work effected on the wafer chamfers W1 by the buff 140 is low, so that the time required for effecting the specular finish polishing is relatively long, and the wafer production efficiency is restricted.
The present invention was contrived in view of this problem, and it is, therefore, an object of the invention to provide an improved method and apparatus for polishing chamfers of a wafer in a manner such that the required polishing time will be reduced and the wafer production efficiency improved.
With this object in mind the inventors conducted a theoretical study of comparing the polishing time required for specular finish in the case of a novel internal surface-contact type buff (convex-to-concave contact) with that in the case of the conventional external surface-contact type cylindrical buff (convex-to-convex contact). The study will be described with reference to FIG. 8(a) and FIG. 9(a); in FIG. 8(a) a wafer W with exaggerated thickness is seen to be inscribed to the internal surface of an internal surface-contact type buff 240, and in FIG. 9(a) a wafer W with exaggerated thickness is seen to be in contact with the external surface of an external surface-contact type cylindrical buff 240.
The rate of reduction (removal) of roughness R of the chamfers of a wafer W is considered to be proportionate to the roughness R itself and to the length L of the area of contact between the wafer W and the buff 240 measured in the direction of brushing (polishing); hence, the following equation holds: EQU dR/dt=-KLR (1)
wherein K is a coefficient determined by the contact pressure, relative velocity at the contact point, slurry condition, etc; t is time during which the polishing is conducted, and L is the length of the contact area.
The above equation is rewritten into a following differential equation: EQU dR/R=-KLdt (2).
And by solving this differential equation, we obtain a following equation, EQU log R/R.sub.0 =-KLt (3)
wherein R.sub.0 is the initial roughness of the chamfers (i.e., the roughness at t=0, or R.sub.t=.spsb.0).
Consequently, the roughness R of the wafer chamfers at any moment is given as a function of polishing time t: EQU R=R.sub.0 e.sup.-KLt ( 4).
According to the theory of elastic contacts, the length L1 of the contact area in the case where the wafer edge is inscribed by the internal polishing surface of an internal surface-contact type buff 240 (convex-to-concave contact), shown in FIG. 8(a), is substantially greater than the length L2 of the contact area in the case where the wafer edge contacts the side surface of the cylindrical buff 240 (convex-to-convex contact), shown in FIG. 9(a); and such result is intuitively confirmable, as shown in FIG. 8(b) and FIG. 9(b) (L1&gt;L2). Also, it is intuitively confirmable that, in the case of convex-to-concave contact, the closer the radius of curvature of wafer to that of the internal polishing surface without exceding it, the greater the length L of the contact.
Values of roughness R of the wafer chamfers at different times t are calculated by means of equation (4) both in the case of convex-to-concave contact and in the case of convex-to-convex contact, and the results are plotted in FIG. 10 to show the variation of the roughness R of the wafer chamfers with time. In FIG. 10, curve a corresponds to the case of convex-to-concave contact (L1) and curve b corresponds to the case of convex-to-convex contact (L2). Suppose it is desired that roughness R is reduced to a low level value of R.sub.A ; then, the polishing time t required to attain roughness R.sub.A is t1 in the case of convex-to-concave contact and t2 in the case of convex-to-convex contact, and t2 is definitely greater than t1. Hence, the time efficiency of polishing is higher in the case of convex-to-concave contact polishing where a long contact length L1 is obtained than in the case of convex-to-convex contact polishing where the contact length L2 is relatively small. Thus, a change to convex-to-concave type polishing from convex-to-convex type polishing will shorten the required polishing time t and contribute to a sharp increase in wafer productivity.
The inventors, therefore, contrived their novel method and apparatus for polishing chamfers of a wafer based on this concept.